A dielectric hollow antenna apparatus includes a hollow inside tapered rod (e.g., a waveguide) with a flat section and a cap. The antenna further includes a feed through section, a feed pin, and a metal flange. A low loss dielectric material fills the hollow rod that protrudes beyond the metal waveguide to form a radiating element. The radiating element is designed in such a way to maximize radiation and minimize reflections over the antenna bandwidth. The feed through section reduces internal reflection and the waveguide is designed to include a rectangular waveguide that support a propagation (TE01) mode and the waveguide then transitions to a circular waveguide that supports another propagation (TE11) mode. The antennas can be employed for radar level gauging and withstand high temperature and possesses a small diameter that permits the antenna to fit in small tank nozzles.